SolarSizing: free calculators, sources on every figure
How many solar panels you need, what they produce and when they pay back, from published sun-hour and cost data.
Three calculators for a home in the US or the UK: how many panels your kWh need, what they produce where you live, and when they pay back at your electricity price. Sun hours from NREL data and PVGIS, losses from NREL PVWatts, prices from EIA and Ofgem, costs from published surveys. No installer quotes, no lead form, nothing stored on our servers.
The calculators
How many solar panels do I need?
From your monthly kWh and your state or city: the system size in kW, the number of 400 W panels and the roof area they take.
Solar payback and savings calculator
Net cost from a published cost per watt, year-one savings at your electricity price, simple payback and the 25-year table with 0.5% a year of output decline.
Solar panel output calculator
kWh per day, month and year for a system size and a place, per panel too, with the split by month where the source publishes one.
How it works
- Type the kWh from your bill and pick your state or UK city. The sun hours, the panel wattage and the losses start from published figures, each with its source and the date it was read; change any of them.
- Read the system size in kW, the number of panels and the roof area, then open the output tool for the kWh per day, month and year, month by month where the source publishes it.
- Put in a quote (or the survey average) and your electricity price: net cost, year-one savings, simple payback and the 25-year table. Nothing is uploaded; Pro prints the three results as one report.
Example with the US starting values: 899 kWh a month in Texas (5.3 peak sun hours, 14% losses) needs 6.48 kW, 17 panels of 400 W on 352 sq ft. Estimates from published averages (NREL, EIA, Ofgem, industry cost surveys), each with its source and date: an installer's site survey and shading analysis replace them. Not financial advice.
From the blog
All articles- How to size a solar system from your electricity bill
Seven steps from the kWh on the bill to a system size, panel count, roof area, year-one output and payback, worked for a Texas home and a London home.
Want it on paper? Pro prints the sizing report.
Inputs, system size, panels and roof area, production by month, cost, payback and the 25-year cash flow, with every source. Print, save as PDF or export to Excel.
Frequently asked questions
- Where do the numbers come from?
- Every starting value shows its source under the field and in the Sources list at the bottom of each calculator: NREL data by state and the European Commission's PVGIS for sun hours, NREL's PVWatts manual for the 14% losses and the 0.5% a year decline, a Trina data sheet for the 400 W panel's size, EIA and Ofgem for electricity prices, EnergySage and the Energy Saving Trust for installed costs, the IRS and HMRC for incentives and VAT. Each carries the date it was read.
- Is this a quote or a sales tool?
- No. There is no lead form and no installer network; the figures are national averages you replace with your own. A site survey and shading analysis by an installer replace the estimate.
- Does it work outside the US and the UK?
- The presets cover US states and UK cities. Anywhere else, pick "Enter sun hours" and type your place's figure (PVGIS covers Europe, Africa and much of Asia; solar maps exist for most countries); the maths is the same. Prices and costs then need your own figures too.
- What does Pro add?
- One printable report with the three results and the sources, production by month, the 25-year cash flow, and an Excel export. The free tools show everything on screen already.